Passive Intermodulation (“PIM”) distortion is a form of electrical interference that may occur when two or more RF signals encounter non-linear electrical junctions or materials along an RF transmission path. Such non-linearities may act like a mixer causing new RF signals to be generated at mathematical combinations of the original RF signals. If the newly generated RF signals fall within the bandwidth of existing RF signals, the noise level experienced by those existing RF signals is effectively increased. When the noise level is increased, it may be necessary reduce the data rate and/or the quality of service. PIM distortion can be an important interconnection quality characteristic for an RF communications system, as PIM distortion generated by a single low quality interconnection may degrade the electrical performance of the entire RF communications system. Thus, ensuring that components used in RF communications systems will generate acceptably low levels of PIM distortion may be desirable.
PIM distortion may be caused by, for example, inconsistent metal-to-metal contacts along an RF transmission path, particularly when such inconsistent contacts are in high current density regions of the transmission path such as inside RF transmission lines, inside RF components, or on current carrying surfaces of an antenna. Such inconsistent metal-to-metal contacts may occur, for example, because of contaminated and/or oxidized signal carrying surfaces, loose connections, metal flakes or shavings inside RF connections and/or poorly prepared RF terminations (e.g., a poor termination of a coaxial cable into a coaxial connector). PIM distortion may arise in a variety of different components of an RF communications system. For example, non-linearities may exist at the interconnections in an RF communications system where cables such as coaxial cables are connected to each other or to RF equipment. PIM distortion may also arise in other components of an RF communications system such as RF amplifiers, duplexers, cross-band couplers, interference mitigation filters and the like. PIM distortion may also arise on or within radiating elements of the RF communications system such as parabolic antennas or phased array antenna elements. The non-linearities that give rise to PIM distortion may be introduced at the time of manufacture, during installation, or due to electro-mechanical shift over time due to, for example, mechanical stress, vibration, thermal cycling, and/or material degradation.
PIM testing is routinely performed to identify components and/or interconnections that exhibit unacceptably high levels of PIM. International standard IEC 62037 sets out acceptable techniques for measuring PIM. Typically, various components of an RF transmission system will be rated to have PIM levels below certain ranges when tested according to the above-referenced international standard. PIM measurements may specify, for example, a magnitude of a third order intermodulation distortion signal. The PIM measurement may also specify operating characteristics of the device under test during the PIM distortion test. For example, for a tower mounted amplifier, the PIM measurement may specify the gain setting of the amplifier during the PIM distortion test.